Device and method for digital-to-analog transformations and reconstructions of multi-channel electrocardiograms

ABSTRACT

The present invention includes a method and apparatus for digital to analog conversion and reconstruction of multichannel electrocardiograms. The method may include receiving digital information representative of a plurality of independent signals, producing a plurality of analog outputs from said digital information wherein a first analog output is designated as a common reference, and imposing a predetermined voltage on a second analog output with respect to said common reference, which provides for a substantial recreation of the original independent signals. The apparatus may comprise a processor operable for receiving digital information representative of independent lead signals from a first ECG machine and digital to analog circuitry for substantially reproducing the original lead signals for analysis on a second ECG machine for convenient and efficient second opinions of cardiac data.

This application claims benefit of U.S. Patent Application provisionalapplication No. 61/720,207 filed Oct. 20, 2012, which is herebyincorporated by reference.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat.435; 42 U.S.C. 2457).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to electrocardiogram systemsand, in one more particular embodiment, to a specialized method and/orsystem for digital to analog conversion (DAC) and reconstruction ofmultichannel electrocardiograms (ECGs), including 12-lead ECGs,compatible with multiple ECG manufacturers.

2. Background

Most modem electrocardiogram (ECG) machines use built-in analog todigital converters (ADCs) to digitize patients' analog cardiacelectrical signals for more efficient analysis, display, storage,printing, and sharing of data. While this common and intuitive methodhas heretofore been sufficient for most clinical uses, this processtypically “locks in” the practicing clinician to the often opaque andsometimes proprietary digital format(s) of the specific ECG machine(s)being employed. In contradistinction, and particularly for patients withdifficult-to-interpret 12-lead ECGs wherein the automated diagnosis fromthe “house machine” may be in question (no automated algorithm beingerror free), many clinicians might welcome the opportunity tooccasionally obtain one or more additional opinions from othermanufacturers' automated interpretive algorithms. Different algorithmsfor example are sometimes known to have widely varying diagnosticaccuracies for common electrocardiographic conditions.

As noted above, this can be problematic because each manufacturer usescompeting proprietary programs to analyze the ECG data, restrictingphysicians and/or technicians to a particular automatic diagnosticalgorithm. These diagnostic algorithms have variances that cause each ofthem to be better suited in diagnosing certain diseases and/or defectsbetter than others.

Furthermore, most ECG machines are only designed to receive analog data,not digital data. Therefore, should a second automated opinion benecessary or advised, a patient must normally be subjected to a secondECG test, because the second ECG machine cannot readily receive thedigital information generated in the first ECG machine to generate asecond analysis. Even though it would be highly desirable to analyzedata utilizing the diagnostic programs of different ECG machinemanufacturers, there is presently no ready solution for ECG datagenerated on Manufacturer X's machine to subsequently be analyzed byManufacturers Y's machine for diagnosis and second opinions.

The following background prior art patents disclose various types of ECGapparatuses, methods and/or systems, but do not address the problemsdiscussed hereinbefore:

U.S. Pat. No. 7,197,357, issued Mar. 27, 2007 to Istvan et al.,discloses a cardiac monitoring system and, more particularly, a wirelesselectrocardiograph (ECG) system. The base station converts the digitalsignals back to analog electrical signals that can be read by an ECGmonitor. However, this art is directed at removing the plethora ofcables associated with ECG machines and not solving the problem ofcompatibility issues between competing ECG manufacturers.

U.S. Pat. No. 6,611,705, issued Aug. 26, 2003 to Hopman et al.,discloses a method and system for wireless ECG monitoring. An electrodeconnector, transmitter and receiver operate with existing electrodes andECG monitors. The electrode connector includes connectors for attachingto disposable or reusable single electrodes. The transmitter transmitsthe signals from the electrodes to the receiver. The receiver passes theelectrode signals to the ECG monitor for processing. ECG monitors usedwith an electrical conductor, for example wire connections toelectrodes, are connected with the receiver. A Legacy ECG monitor isavailable to connect with the receiver using the ECG monitor'slead-wires. The ECG monitor operates as if directly connected to theelectrodes without wires running from the ECG monitor to the patient.

U.S. Pat. No. 3,810,102, issued May 7, 1974 to Parks III, et al.,discloses a method and system for transmitting biomedical data to aremote station for subsequent processing. Analog electrical biomedicalsignals are sampled and digitized at a relatively low data rate andtransmitted over a communications link of limited bandwidth to a remotestation where the analog electrical biomedical signals are reconstructedfrom the digital data and are sampled and digitized at a substantiallyhigher data rate for subsequent interpretation by a diagnostic computer.Alternatively, the received digital data are directly converted to asubstantially higher digital data rate by means of a numericalalgorithm, a form of digital interpolation.

U.S. Pat. No. 8,082,027, issued Dec. 20, 2011, to Young, et al.,discloses a system and method of the present application comprising ECGacquisition device having a USB connector for connecting the device to ahost device and a patient connector for connecting the device to apatient with ECG leads. The ECG acquisition device of the present systemfurther includes a processor and storage medium, a power management andbrokering module, a USB communications control module, an ECGacquisition circuit, and a patient isolation module. The ECG acquisitiondevice auto-loads and runs ECG monitoring software onto the host device.

U.S. Pat. No. 7,783,339, issued Aug. 24, 2010, to Lee, et al., disclosesa method and system for real-time digital filtering forelectrophysiological and hemodynamic amplifiers. The invention replacesthe analog circuits currently used for signal filtering and conditioningin such systems with digital filters that may be implemented in asoftware application. The method and system includes digitizing theanalog signal collected from the patient prior to performing the signalfiltering and conditioning. The method and system also includes removingstimulus artifacts, as well as performing sample rate conversion andscaling on the digital signal. The processed digital signals may beused, displayed, saved and converted to analog signal throughdigital-to-analog conversion.

U.S. Pat. No. 7,184,921, issued Feb. 27, 2007, to Kohls, discloses atechnique for encoding physiological data, such as a digital ECG, as aset of high-resolution symbols. The set of high-resolution symbols maybe printed on a printout of the physiological data or other suitablemedium. The set of high-resolution symbols may be scanned, or otherwiseacquired, and decoded to reconstruct all or a portion of the originalset of physiological data.

U.S. Pat. No. 6,735,464, issued May 11, 2004, to Onoda, et al.,discloses an electrocardiograph system having an electrocardiographtransmitting cardiograms produced to outside equipment, and acommunication device wirelessly communicating with theelectrocardiograph. The communication device accepts a subject's postureselected from multiple posture options, and transmits a specificinstruction signal to the electrocardiograph upon receiving theselection. The electrocardiograph stores a cardiogram produced when theinstruction signal is received from the communication device as areference cardiogram corresponding to the selected posturediscriminating it from other cardiograms.

U.S. Patent Application No. 20100017471, issued Jan. 21, 2010, to Brown,et al., discloses systems and methods for providing improved medicalcare. A system includes a defibrillator, a gateway device, a routingdevice, and a wireless modem. The system may further include hardwareand/or software components located at a remote facility for receivingdata and one or more server devices for decoding data from the remotefacility. A method includes acquiring medical data at a first location,converting the medical data from an analog signal to a digital signal,transmitting the digital signal from the first location to a secondlocation over the internet via a cellular network, receiving the digitalsignal at the second location, and converting the digital signal back toan analog signal for processing. The first location may be an EMSvehicle, and the second location may be a remote facility, such as adispatch center or local hospital.

There exists a need for a specialized method and system for digital toanalog conversion (DAC) and reconstruction of multichannelelectrocardiograms (ECGs) which addresses the problems associated withthe prior art described hereinbefore. The present invention has directcommercial, military, and/or medical applications. Furthermore, thepresent invention would be valuable not only for further review of ECGdata and second opinions, but also for improving diagnostic algorithmsamongst ECG manufacturers through collaboration. Consequently, thoseskilled in the art will appreciate the present invention that addressesthe above and other problems.

SUMMARY OF THE INVENTION

One possible object of the present invention is to provide a specializedsystem for digital to analog conversion (DAC) and reconstruction ofmultichannel electrocardiograms (ECGs), including 12-lead ECGs. The wordlead has multiple meanings in electrocardiography: for example, itrefers to either the wire that connects an electrode to theelectrocardiograph, or (more commonly) to a combination of electrodesthat form a virtual line in the body along which the electrical signalsare measured. Thus, the term loose lead artifact uses the formermeaning, while the term 12-lead ECG uses the latter. In fact, a 12-leadelectrocardiograph usually only uses 10 wires/electrodes.

Another possible object of the present invention is to provide aspecialized system for digital to analog conversion (DAC) andreconstruction of multichannel electrocardiograms which can interfacewith multiple different ECG machines.

An additional possible object of the present invention is to provide aspecialized electrocardiogram DAC and reconstruction system whichprovides for less expensive and/or less bulky front end ECG hardware forremote and impoverished areas.

Yet another possible object of the present invention is to provide aspecialized electrocardiogram DAC and reconstruction system whichprovides for rapid automated second opinions of difficult to interpret12-lead ECGs.

Yet another possible object of the present invention is to provide aspecialized electrocardiogram DAC and reconstruction system whichprovides for improved performance of automated ECG interpretations,e.g., when ECG machines from multiple manufacturers are used in the samesetting.

Yet another possible object of the present invention is to provide aspecialized electrocardiogram DAC and reconstruction system which canfurther the clinical utility of technology that converts paper ECGprintouts to digital ECG files.

These and other objects, features, and advantages of the presentinvention will become clear from the figures and description givenhereinafter. It is understood that the objects listed above are not allinclusive and are only intended to aid in understanding the presentinvention, not to limit the bounds of the present invention in any way.

In accordance with the present invention, a specialized method andapparatus for digital to analog conversion and reconstruction ofmultichannel electrocardiograms is disclosed which may comprise stepssuch as, for example, receiving digital information representative of aplurality of independent signals (i) between a predetermined at leastone pair of a plurality of electrodes placed on a living creature or(ii) between at least one individual electrode from said plurality ofelectrodes placed on a living creature and a predetermined referencecomprising an electrical resultant or potential difference between twoor more predetermined electrodes of said plurality of electrodes placedon a living creature. As used herein, a “living creature” comprises atleast a plurality of limbs and a chest. For example, a living creaturemay be a human being having four limbs (i.e., two arms and two legs anda chest). As another example, a living creature may be a cow having fourlimbs (i.e., four legs and a chest). Many other examples exist. Inaddition, in an embodiment, a predetermined reference may be comprisedof a predetermined two or more of said plurality of independent signals.Still further, in an embodiment, a predetermined reference may representan average value of a predetermined two or more of said plurality ofindependent signals.

Other steps may comprise producing a plurality of analog outputs fromthe digital information wherein a first analog output is designated as acommon reference, and imposing a predetermined voltage on a secondanalog output with respect to the common reference.

The method may further comprise utilizing the plurality of analogoutputs for substantially recreating lead signals originally produced byplacing the plurality of electrodes on the living creature whichresulted in creation of the digital information.

Other steps of the method may include receiving the digital informationin a predetermined format and converting the digital information in thepredetermined format to a predetermined optimal format.

In an embodiment, the predetermined format may be based on utilizing anelectric potential involving two or more electrodes (i.e., a group ofelectrodes) of the plurality of electrodes as a signal reference. Thus,in this embodiment, the step of converting the digital information froma predetermined format to an optimal format may comprise a step ofassigning a common reference to a first analog output wherein the firstanalog output corresponds to a limb electrode. In an embodiment, thestep of assigning may be executed in a digital to analog (DAC) device.

In another possible embodiment, a step of converting the digitalinformation from a predetermined format to an optimal format maycomprise assigning the common reference to a first analog output whereinthe first analog output corresponds to a particular one electrode (i.e.,a first electrode) of the plurality of electrodes. In anotherembodiment, the method may further comprise a step of utilizing DAChardware (i.e., a DAC device) to perform the step of converting thedigital information.

In one embodiment, the plurality of electrodes may comprise a pluralityof limb electrodes and a plurality of precordial electrodes configuredto produce an ECG recording. However, the invention is not limited toECG recordings and may comprise other types of living creature electroderecording devices.

In one embodiment, the plurality of limb electrodes may comprise a rightarm electrode, a left arm electrode, a right leg electrode, a left legelectrode, or any combination thereof. A second analog output discussedabove may correspond to another particular one electrode (i.e., a secondelectrode) of the plurality of electrodes comprising a second limb ofthe living creature. The analog outputs generally correspond to and maybe considered as corresponding electrodes.

In another embodiment of the present method, the step of imposing apredetermined voltage on the second analog output with respect to thecommon reference may comprise imposing a zero voltage on the secondanalog output. Imposing a zero voltage on the analog output may be donewith software or hardware and may comprise directly connecting thesecond analog output to the common reference. In one embodiment, themethod of directly connecting comprises connecting the second analogoutput to the common reference through a resistor, which is utilized forimpedance matching, as might be used for impedance matching on an ECGmachine into which the analog signals could be fed.

In one embodiment, the plurality of electrodes comprises a plurality oflimb electrodes and a plurality of precordial electrodes configured toproduce an ECG recording, and the plurality of analog outputs correspondto the plurality of limb electrodes and the plurality of precordialelectrodes and the digital information is produced by a first ECGmachine made by a first manufacturer, and further comprising connectingthe plurality of analog outputs to a second ECG machine made by a secondmanufacturer.

In another embodiment, the method may comprise comparing a firstanalysis of the ECG recording produced by the first ECG machine to asecond analysis of the ECG recording produced by the second ECG machine.The method may further comprise utilizing the plurality of analogsignals to produce a plurality of analyses from different ECG machinemanufacturers.

In an alternative embodiment of the present invention, an apparatus isprovided for recreating lead signals that are substantially equivalentto those original signals produced by placing a plurality of electrodeson a living creature. Another definition of “lead” is an output from apair of electrodes. A processor is operable for receiving digitalinformation representative of independent lead signals, wherein eachindependent lead signal represents at least one of electrical potentialsbetween pairs of the plurality of electrodes or electric potentialsbetween individual electrodes of the plurality of electrodes and areference representing signals, such as electric potentials, from atleast two electrodes (i.e., a group of electrodes) of the plurality ofelectrodes.

Other elements may comprise digital to analog circuitry (which may ormay not be incorporated in a digital to analog conversion hardware ordevice) with a plurality of analog outputs wherein one analog output isdesignated as a common reference and wherein a predetermined voltage isimposed on another of the analog outputs. The processor and digital toanalog and/or other components may be discrete components, integratedcomponents, firmware, and/or may be designed in optimal fashion asdesired.

In this embodiment, the recreated substantially original lead signalscomprise electrical potentials between the plurality of analog outputs.

The apparatus may further comprise a processor operable for receivingthe digital information in a predetermined format and converting thedigital information in the predetermined format to a predeterminedoptimal format. The processor is then operable and/or programmed tocontrol the digital to analog conversion hardware so that the commonreference is the first analog output, and wherein the first analogoutput corresponds to a particular one of the plurality of electrodes.

In one embodiment, the received digital information from an ECG machineutilizes electric potentials between pairs of the plurality ofelectrodes (e.g., for leads I, II and III) and electric potentialsbetween an individual electrode of the plurality of electrodes as wellas a reference electric potential involving a group of the plurality ofelectrodes such as, for example, Wilson's Central Terminal (e.g., forleads V1-V6) or one side of Einthoven's triangle (e.g., for leads aVR,aVL and aVF) whereas in the predetermined optimal format the system isoperable for first controlling the digital conversion process and thenthe digital to analog conversion process to change the referenceelectric potential from any group of electrodes to the first analogoutput.

In an embodiment, a particular one electrode (i.e., a first electrode)of the plurality of electrodes could be an electrode placed on a limb ofthe living creature. In another embodiment, the plurality of electrodesmay comprise a plurality of limb electrodes. The plurality of limbelectrodes may comprise a right arm electrode, a left arm electrode, aright leg electrode, a left leg electrode, or any combination thereof.The second analog output may correspond to another particular oneelectrode (i.e., a second electrode) of the plurality of electrodescomprising a second limb of the living creature.

In another embodiment, the predetermined voltage on the second analogoutput with respect to the common reference may or may not comprise azero voltage. The imposition of a zero voltage to second analog outputcomprises a direct connection from the second analog output to thecommon reference. The direct connection comprises a connection from thesecond analog output to the common reference through a resistor utilizedfor impedance matching.

In one embodiment, the plurality of electrodes comprises a plurality oflimb electrodes and a plurality of precordial electrodes configured toproduce an ECG recording, and the plurality of analog outputs correspondto the plurality of limb electrodes and the plurality of precordialelectrodes. The digital information is produced by a first ECG machinemade by a first manufacturer, and further comprising a plurality ofconnections connecting the plurality of analog outputs to a second ECGmachine made by a second manufacturer.

In one embodiment, a first ECG machine is operable for producing a firstanalysis of the ECG recording, and a second ECG machine is operable forproducing a second analysis of the ECG recording. In other words, theplurality of analog outputs is connectable to produce a plurality ofanalyses from different ECG machine manufacturers.

Another embodiment of the present invention may include a method foranalyzing data produced by a first ECG machine that utilizes a pluralityof electrodes and a reference comprising signals from at least two(i.e., a group) of the plurality of electrodes to produce a plurality oforiginal lead signals, comprising receiving digital data from the firstECG machine and reconstructing a plurality of analog signals from thedigital data from which can be obtained substantially identical leadsignals as compared to the original lead signals produced by the firstECG machine, wherein a first of the plurality of analog signalscomprises a new reference, and wherein an average RMS or other voltageor amplitude difference between the plurality of original lead signalsand the substantially identical lead signals is calculated.

The method may comprise utilizing the plurality of the analog signalsfobr producing an analysis of the substantially identical lead signalsby a second ECG machine. The first of the plurality of analog signalscorresponding to a first limb electrode. The method may further compriseimposing a predetermined voltage on a second of the plurality of analogsignals and the second of the analog signals corresponds to a secondlimb electrode. In one possible embodiment, the predetermined voltage iszero with respect to the new reference.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention and many of the attendantadvantages thereto will be readily appreciated as the same becomesbetter understood by reference to the following detailed descriptionwhen considered in conjunction with the accompanying drawings, wherein:

FIG. 1 is a functional diagram of a standard 12-lead electrocardiogramsystem, referenced to Wilson's Central Terminal, as utilized in typicalECG configurations.

FIG. 2 is another functional diagram of a 12-lead electrocardiogramsystem as utilized in a typical ECG configuration.

FIG. 3 is a schematic diagram of the digital to analog conversion andanalog to digital conversion of ECG data in accord with one possibleembodiment of the present invention.

FIG. 4 is a device that may be utilized as part of reconstruction ofanalog signals from digitized signals of different ECG machinemanufacturers in accord with one embodiment of the present invention.

FIG. 5 is an original ECG report as described hereinafter.

FIG. 6 is a reconstructed ECG report of the original ECG report of FIG.5 in accord with one possible embodiment of the present invention.

FIG. 7 is a second original ECG report as described hereinafter.

FIG. 8 is a reconstructed ECG report of the original ECG report of FIG.5 in accord with one possible embodiment of the present invention.

The above general description and the following detailed description aremerely illustrative of the generic invention, and additional modes,advantages, and particulars of this invention will be readily suggestedto those skilled in the art without departing from the spirit and scopeof the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention comprises optimized methods and hardwareconfiguration that accurately and effectively convert digital ECG datacollected on one ECG machine manufacturer's hardware to analog, and thenreconstructs the analog multichannel ECG data for insertion into anothermanufacturer's ECG machine for automated second opinions.

Referring generally to FIG. 3, which is discussed in more detail below,the present invention is designed to begin with digital data, stored,streaming, or scanned in from ECG printouts as indicated at 300, thathas been converted to an open digital format (provided hereinafter)optimized for the task of reproducing nearly identical original analogECG signals. Once the original analog signals are reproduced, the systemcan move those signals forward into any manufacturer's ECG machine(s) tobe re-digitized (or “reconstructed”) there as indicated at 340. Thusautomated diagnostic interpretations from multiple manufacturers' ECGmachines can be obtained for any single ECG data file or stream, eitherlocally or remotely, and with any desired degree of fidelity dependentonly on the specifications of the “front end” ADC used for the originaldata collection.

From a practical standpoint, the described system will facilitate theconstruction and use for data collection of very inexpensive 12-lead ECGhardware front ends at remote sites including for example by patients athome or by third world practitioners who can use inexpensive cell phoneor tablet devices to rapidly forward digital data to (and receiveautomated reports back from) remote server or cloud-based analyticalsites. Because the ECG machines most commonly utilized in Westerncountries today are usually cost-prohibitive to employ in a “commodity”fashion, the use of the system described herein might result insignificant cost savings to both governmental medical funding entitiesand humanitarian organizations. Under such a scenario, theaforementioned more expensive and bulky traditional ECG machines couldinstead be employed merely singly on the “back end”, specifically at anECG telemedicine central server site or within a cloud-based equivalent.

The system described herein was also specifically designed forprocessing ECG data collected in certain remote places wherein the mainlimitation may not be cost per se but rather the amount of allowablemass/volume and/or of locally available expertise, for example for12-lead ECG data arriving from space or from remote terrestrialenvironments such as mobile military units, oil platforms ormountaineering, polar or other expedition areas. The present inventionis not limited to 12 lead ECG machines or ECG machines in general but isapplicable to other types of machines that involve collection of signalsfrom electrodes placed on living creatures.

Referencing FIG. 1, a functional diagram of a typical 12-leadelectrocardiogram system 100 is depicted. When collecting standard12-lead ECG 100, ten electrodes placed on the patient are used to obtainup to 9 different analog voltages, which are sometimes referred to asleads. These are voltages measured between various electrodes asdiscussed below. For a standard 12-lead ECG, ten electrodes may beplaced on a living creature, which include limb electrodes 62 andprecordial electrodes 50. Various types of ECG systems and other systemsfor measuring voltages from machines may be utilized in accord with thepresent invention. However, the present invention is described in termsof the standard 12-lead ECG. FIG. 2 shows another simplified 12 lead ECGsystem wherein the multiplexor and analog to digital signal converterare shown.

The limb electrodes 62 may comprise left arm electrode 26, right armelectrode 28, left leg electrode 30, right leg electrode 32, or anycombination thereof. It is of note that in this situation right legelectrode 32 is used only as a voltage common for generating the nineindependent analog voltages. Additionally, precordial electrodes 50 aretypically placed surrounding a patient's chest and include 6 electrodes,i.e., EC1 electrode 34, EC2 electrode 36, EC3 electrode 38, EC4electrode 40, EC5 electrode 42, and EC6 electrode 44.

The measured voltage differences (i.e., signals) between the electrodesare stored digitally as independent data leads 60, which most ofteninclude lead I 10, lead II 12, and leads V₁-V₆, 14-24, as referenced toWilson's Central Terminal, or WCT 46 wherein the Wilson's CentralTerminal is commonly known in the art. If one defines the original tenelectrodes as follows: left arm=EL; left leg=EF; right arm=ER; rightleg=N (reference neutral) and chest=EC_(i)(i=1-6), then the independentdata leads 60 would most commonly be expressed as:

I=EL−ER;

II=EF−ER;

V _(i) =EC _(i)−WCT;

WCT=(EL+ER+EF)/3.

As discussed herein, ECG machines are typically only capable ofreceiving analog data to operate as intended. Once the information(i.e., signals) is stored digitally as outlined above, the problemencountered becomes how the eight independent data leads can be utilizedto produce at least nine DAC channels, once uncoupled from WCT 46provided on the original ECG machine (because the final receiving ECGmachine is itself again expected do such coupling), that will producedata lead I 10, data lead II 12, and data leads V₁-V₆, 14-24, at thereceiving ECG machine. To solve the problem, the reconstructed leadsignals should appear as if the DAC channels had come from limbelectrodes 62 and precordial electrodes 50 used on a patient to generatethese data channels, essentially simulating the measurements that werepreviously made on a living creature, such as a live patient. In anembodiment and in accord with the present invention, as explainedherein, for a 12-lead ECG system that utilizes a digital file formatwherein the chest electrodes are referenced to WCT, a digitaltransformation is first performed wherein the chest electrodes areinstead referenced to the right arm electrode (i.e., to ER, therebyproducing “CRi” instead of “Vi” chest lead data).

FIG. 1 expresses the problem graphically by showing a generalizedfunctional diagram for a typical 12-lead ECG system, but ignoring (asdefined) the non-independent leads III, aVR, aVL and aVF.

In one possible embodiment of the present invention, for any 12-lead ECGsystem that utilizes a digital format, the format is first changed ifnecessary to one in which the chest electrodes are referenced not toWCT, but instead to an electrode, such as a limb electrode. In onepossible example, the right arm electrode is utilized.

Accordingly, with reference to right arm electrode 28 (i.e., to ER,thereby perhaps better distinguished in terminology by producing“CR_(i)” instead of “V_(i)” chest lead data), the following applies:

CR _(i) =EC _(i) −ER;

Referring to FIG. 4, DAC (see generally DAC 402 in FIG. 4) receives theoptimally formatted digital data from control 404 for representativedigital data to provide a means to convert back to substantiallyoriginal leads.

In one possible example, a “zero” voltage can be imposed upon its rightarm electrode input, i.e., what might be termed “ER0”. A possibleexample of this is seen in circuit form of FIG. 4, where analog output406, which is conveniently labeled RA, but corresponds to ER, is tied tozero. The zero voltage of RA could be imposed by software but in thiscase is done in hardware. Resistor 412 is provided for impedancematching purposes. So at this time the analog signal RA is zero voltswith respect to the analog output neutral 408, conveniently referred toas RL that corresponds to the right leg electrode, that is provided asthe neutral or ground as indicated at 410. So now the algebra discussedabove is transformed in a way that allows substantially original leadsignals to be reconstructed. Specifically, going through the algebraicdescription again so that RA has been tied to zero with respect to theneutral:

I=EL;

II=EF;

CR _(i) =EC _(i);

and WCT=(1+II)/3.

Therefore, if the following conditions are assigned to DAC 402, theyshould ultimately produce, on most if not all ultimately receiving(re-digitizing) 12-lead ECG machines, the desired I, II, and V₁-V₆ datalead signals:

-   -   0 volts on the right arm electrode input ER (also referred to as        RA);    -   Lead I signal on the left arm electrode input EL;    -   Lead II signal on the left leg electrode input EF;    -   DAC common on the right leg electrode input N;    -   CR_(i) signals derived from V_(i) channels on the precordial        electrode inputs EC_(i).

In other words, referring to analog outputs 414 in FIG. 4, the leadsignals can be recreated from the ten analog outputs, which areconveniently labeled as they would be if they were actual electrodes onthe living creature.

Other aspects of the present invention are of interest to those skilledin the art. First, in the example provided, the chest electrodes arereferenced not to WCT, but instead to the right arm electrode, a systemoriginally favored by original persons of interest in the ECG field,such as Einthoven. Other persons in the 12 lead ECG field also favoredsuch a reference, even after the introduction of WCT in the 1930s. Whilethe above is one embodiment of the invention, it is also possible toaccomplish the same fundamental end point through a digital formatwherein all other electrodes are referenced to the left arm electrodewhile a zero voltage is simultaneously imposed on the DAC 402 left armelectrode input, or through a digital format wherein all otherelectrodes are referenced to the left leg electrode while a zero voltageis simultaneously imposed on the DAC left leg electrode input.Accordingly, the present invention is not limited to the particularassignments of neutral and/or application of zero volts to analogoutputs representative of electrode signals as described above.Additionally, while tying analog output 406 to the neutral as describedabove for imposition of zero volts, which presently appears convenient,conceivably other predetermined voltages might also be utilized. So theinvention is not limited to the presently described embodiment. With theten electrode signals recreated, it is now possible to produce thereconstructed lead signals.

However, this format is not exclusive for the present invention tofunction as intended. As described below, two alternate referenceelectrode scenarios are mathematically depicted supposing, for example,that EL or EF is used as a reference instead of ER.

With EL as the reference, then:

CR=ER−EL=−I

CF=EF−EL=III;

CC _(i) =EC _(i) −EL;

II=CF−CR=III+1;

V _(i) =EC _(i)−(ER+EF+EL/3);

V _(i) =CC _(i)−(CR+CF)/3.

With EF as the reference, then:

CL=EL−EF=−III;

CR=ER−EF=−II;

CC _(i) =EC _(i) −EF;

I=CL−CR;

V _(i)=(3EC _(i) −ER−EL−EF)/3;

V _(i) =EC _(i)−(CR+CL)/3.

As discussed in more detail hereinafter, FIGS. 5 and 7 show originalsignals and FIGS. 6 and 8 show the reconstructed lead signals. It willbe appreciated that visually the signals look substantially identical.

Various types of validation studies have been made.

Ten 12-lead ECG data files, each between 5-10 min in length, that werepreviously collected from five healthy and five diseased patients,respectively, using a high-fidelity, 1000 samples/sec 12-lead PC-ECGdevice (Cardiax, IMED Ltd., Budapest, Hungary), were randomly selectedfrom a large set of pre-existing files to help validate the system. The12-lead ECGs were clinically normal in each of the five healthypatients, whereas in the five diseased patients, the followingelectrocardiographic conditions were respectively selected (fromaffected individuals chosen at random) in order to include a wide rangeof electrocardiographic pathologies: 1) Coronary artery disease withoutprior myocardial infarction and with normal QRS interval; 2) Coronaryartery disease with prior myocardial infarction (i.e., ischemiccardiomyopathy) but with normal QRS interval; 3) Non-ischemic (dilated)cardiomyopathy with normal QRS interval; 4) Left bundle branch block ofuncertain etiology; and 5) Right bundle branch block of uncertainetiology.

Validation studies were performed to compare the original digital ECGdata to their reconstructed (i.e., after DAC and repeat ADC) counterpartdata including quantitative and qualitative studies. Quantitativevalidation studies compared the total-waveform voltage differencesbetween the original and reconstructed data while the second typequalitatively compared the automated electrocardiographic (i.e.,clinical) diagnostic statements generated by the original versusreconstructed data.

For quantitative validation, a MATLAB®-based script was written tosuperimpose the data in the original and reconstructed files for eachsubject by using the corresponding R-wave fiducial point locations inthe files to align the corresponding waveforms. Each test file had250-500 PQRST complexes within a 5-10 min data epoch. For each PQRSTcomplex, a region about the R-wave fiducial point was detrended and theoriginal versus reconstructed waveforms were overlaid and shifted tominimize the root mean square (RMS) difference. The standard deviationwas used as a proxy because detrending alone ensures a near zero meanbut not a perfectly zero mean. An average RMS difference estimate acrossall beats was then calculated for each channel in each patient, as wasan overall average RMS difference for all channels combined. This sameprocess was performed twice: once after having used the same model ofADC (Cardiax 12-lead ECG recorder) to collect the reconstructed(re-digitized) data that had also been used to collect. the originaldata; and once after having used a different manufacturer's ADC recorder(BT 12, CorScience, Erlangen. Germany) to collect the re-digitized data.

TABLE 1 Channel: Subject I II CR1 CR2 CR3 CR4 CR5 CR6 1H 5.2 6.6 5.814.4 10.4 9.9 9 8.4 2H 2.8 6.8 4.2 13.6 9.1 9.6 8.3 8 3H 3 7.4 4.1 8 7.111.7 10.8 9.6 4H 7.1 8 7.4 13.2 9.9 9.2 9.4 9.2 5H 3.4 5.3 2.8 8.3 10.78.7 6.8 5.8 1D 2.2 3.5 3.3 6 8 5.1 4.7 4.7 2D 4.2 3 7.1 6.5 4.2 4.2 4.34.6 3D 3.2 3.2 5.6 8.3 7 5.7 5.5 5.5 4D 8.9 10.7 18.2 29.6 23 13.2 11.417 5D 12.5 13.8 9.7 13.9 15.1 15.2 16.7 14.7

For the quantitative validation, voltage comparisons are made. Table 1shows the estimated RMS difference values for each of the eightindependent ECG channels (PQRST) when the same model of ADC recorder(Cardiax) that had been used to collect the original data was also usedto collect the re-digitized data. Under these circumstances, thegrand-average (SEM) RMS difference value between the original andre-digitized data was 8.5±0.05 ADC counts per channel, or equivalently20.8±0.12 microvolts.

TABLE 2 Channel: Subject I II CR1 CR2 CR3 CR4 CR5 CR6 1H 5.3 6.9 6.210.4 9.5 9.5 9.2 8.6 2H 6.6 11.1 8.6 12.8 17.6 13.6 12.3 11.4 3H 8.3 9.78.7 15.7 11.3 13.6 12 11.2 4H 4.6 6.6 6 10.6 8.1 8.5 8.6 7.6 5H 6.3 9.47.1 9.9 12.7 11.1 9.8 8.7 1D 6.2 8.9 8.1 11.2 10.8 10.6 9.8 8.5 2D 11.511.6 13.2 13.9 15.2 15.7 15.4 13.2 3D 6.3 5.9 6.1 8.5 9 10.6 10.8 8.4 4D10.8 18.3 21.5 37.3 27.8 17.3 19.4 26.3 5D 12.7 14.5 11.6 13.5 15.4 16.418.1 15.4

Table 2 shows the estimated RMS difference values for each of the eightindependent ECG channels (PQRST) when the re-digitized data were insteadcollected on an ADC (i.e., CorScience) that was different from the ADC(Cardiax) used to collect and store the original data. Under thesecircumstances, the grand-average RMS difference values between theoriginal and re-digitized data was 11.6±0.08 ADC counts per channel, orequivalently 28.4±0.21 microvolts.

As can be surmised from Tables 1 and 2, there were no clear trends inthe differences generated by the original versus re-digitized files inthe healthy versus diseased subjects when the QRS interval was within aclinically normal range. However, as might be expected, the presence ofeither left (subject 4D) or right (subject 5D) bundle branch block,wherein the QRS interval is relatively prolonged and the total voltagerelatively increased, tended to increase the differences between thevoltages in the original versus re-digitized files.

A more qualitative (clinical) validation was also performed to furthervalidate system performance. The qualitative validation involvesautomated clinical diagnostic systems used by ECG machine manufacturer.Specifically, the automated diagnostic statements produced by commercialelectrocardiographic software for the data within the first 15 secondsin the original files were compared in each case to the automateddiagnostic statements produced for the same data in the post-DACre-digitized files. Such analyses of potential changes in automateddiagnostic statements were in turn performed in three separate ways: 1)by using the automated diagnostic program native to the Cardiax softwareprogram when the Cardiax model of ADC recorder had been used to collectboth the original and re-digitized data; 2) by using the well-validatedLeuven automated diagnostic program for both the original data and there-digitized data when the Cardiax model of ADC recorder had been usedto collect both the original and re-digitized data; and 3) by againusing the Leuven automated diagnostic program for both the original dataand for the re-digitized data when the Cardiax recorder had been used tocollect the original data but the CorScience recorder the re-digitizeddata.

TABLE 3 Subject Original File Re-digitized File 1H No signs ofabnormalities given the patient's No signs of abnormalities given thepatient's age. age. 2H Sinus rhythm; 1 premature sinus complex. Sinusrhythm; 1 premature sinus complex. 3H Corresponds to the followingpathological Corresponds to the following pathological abnormality:undetermined rhythm abnormality: undetermined rhythm 4H No signs ofabnormalities given the patient's No signs of abnormalities given thepatient's age. age. 5H No signs of abnormalities given the patient's Nosigns of abnormalities given the patient's age. age. 1D Sinus rhythm;suggests the following possible Sinus rhythm; suggests the followingpossible abnormality: left atrial enlargement. abnormality: left atrialenlargement. 2D Sinus rhythm; corresponds the following Sinus rhythm;corresponds the following possible abnormality: first-degree AV blockpossible abnormality: first-degree AV block (Long PQ); undeterminedvariation: T wave (Long PQ); undetermined variation: T wave abnormalityabnormality 3D No signs of abnormalities given the patient's No signs ofabnormalities given the patient's age. age. 4D Sinus rhythm; suggeststhe following possible Sinus rhythm; suggests the following possibleabnormality: left bundle branch block. abnormality: left bundle branchblock. 5D Sinus rhythm; suggests the following possible Sinus rhythm;suggests the following possible abnormality: right bundle branch block.abnormality: right bundle branch block.

Table 3 shows the automated clinical diagnostic statements outputted bythe commercial Cardiax software program for all ten cases when both theoriginal and re-digitized files were collected on the same model ofCardiax ADC. As can be surmised from Table 3, for all ten cases, underthese circumstances, there were no differences in the clinicaldiagnostic statements outputted by Cardiax for the original versus there-digitized files.

TABLE 4 Subject Original File Re-digitized File 1H Sinus arrhythmia;normal morphology Sinus arrhythmia; normal morphology 2H Sinusbradycardia; normal morphology Sinus bradycardia; abnormalre-polarization, possibly non-specific; QRS within the normal limits 3HSinus arrhythmia; abnormal re-polarization, Sinus arrhythmia; abnormalre-polarization, possibly non-specific; QRS within the normal possiblynon-specific; QRS within the normal limits limits 4H Sinus bradycardia;normal morphology Sinus bradycardia; normal morphology 5H Normal sinusrhythm; normal morphology Normal sinus rhythm; normal morphology 1DNormal sinus rhythm; normal morphology Normal sinus rhythm; normalmorphology 2D Sinus rhythm with first-degree AV block; left Sinus rhythmwith first-degree AV block; left atrial hypertrophy; abnormal re- atrialhypertrophy; abnormal re-polarization, polarization, possiblynon-specific; QRS possibly non-specific; QRS within the normal withinthe normal limits limits 3D Normal sinus rhythm; possible inferiorNormal sinus rhythm; possible inferior infarction, possibly oldinfarction possibly old 4D Sinus rhythm; complete left bundle branchSinus rhythm; complete left bundle branch block block 5D Sinus rhythm;ventricular extrasystole(s); Sinus rhythm; ventricular extrasystole(s);ventricular extrasystole(s) with full ventricular extrasystole(s) withfull compensation; complete right bundle compensation; complete rightbundle branch branch block block

Table 4 shows the automated clinical diagnostic statements outputted bythe commercial Leuven software program for all ten cases when theoriginal files were collected on the Cardiax ADC and when there-digitized files were collected on either the Cardiax or CorScienceADC (i.e., the ultimate interpretive results from the Leuven programwere the same under both of the above circumstances). Under either ofthese circumstances, the automated diagnostic statements outputted bythe Leuven program for the original versus the re-digitized filesdiffered for only one case (i.e., for healthy patient 2H). Specifically,within the Leuven program, criteria for “abnormal re-polarization,possibly non-specific” were triggered for patient 2H's re-digitized filewhereas such criteria were not triggered for this same patient'soriginal file. While it is unclear whether this minor difference in theLeuven algorithm's automated interpretation would have made any clinicaldifference (it is suspected this is not the case), the original andre-digitized ECGs for this patient as interpreted by the Leuvenalgorithm are shown in FIG. 5 and FIG. 6. Both FIG. 5 and FIG. 6 (whichshows the corresponding “worst-case comparison” between original andre-digitized files as quantified by the greatest differences in RMSvalues; patient 4D) also aptly demonstrate the very minor differencesthat typically occurred between all original versus re-digitized fileswith respect to the various electrocardiographic axes, intervals andvoltages that were outputted by the automated interpretive software.

These results suggest that the system described herein can reproduceoriginal analog signals from stored 12-lead ECG data files with a degreeof fidelity likely sufficient for most clinical applications. Theoverall greater utility and flexibility, more open format, and“readiness for cloud computing” of the system of the present inventionpotentially open up several new avenues for more widespread use of DACdevices in clinical electrocardiography. Specifically, systems such asthose presented herein might eventually allow for all of the following:

1) rapid second opinions from any number of automated interpretiveprograms, e.g., for difficult-to-interpret 12-lead ECGs and rhythms (notonly locally, but also from dedicated remote central servers or “thecloud”);

2) use of less expensive (i.e., commodity-style) 12-lead ECG front ends(ADC hardware) in impoverished or underserved areas, since subsequentDAC permit use of any preferred (or any otherwiseprohibitively-expensive) ECG machine or interpretive program onlysingly, on the “back end”;

3) use of less bulky ECG front ends during space flight or in remoteterrestrial environments such as military mobile units, oil platforms orin mountaineering, polar or other expedition areas;

4) improved performance of all automated ECG analytical softwareprograms through the implementation by manufacturers of those“interpretive lessons learned” that will be more rapidly ascertainableto them both through internal testing and through objective competitionsenabled by the DAC;

5) better within-hospital consistency of automated ECG interpretations,e.g., when ECG machines from multiple different manufacturers are usedin any single institution;

6) better across-study consistency when large digital ECG databases areanalyzed in epidemiological studies, since DAC should theoreticallyallow for the same analytical programs to be used, when desired, acrossall such large studies, even when different collaborating groups don'tall possess the same hardware and software; and, finally for

7) furthering the potential clinical and archival utility of othertechnology that converts paper ECG printouts to digital ECG files.

Of note, the only prerequisite for the use of the described system isthat the format of the original digital data should be known—i.e., topermit conversion into an optimal digital format for DAC—or, if notknown, then alternatively convertible to that optimal format by anintegrated or secondary software program tailored to making suchconversions. Once the data are in an optimized format, then the Systemcan be easily employed either locally or remotely to convert the digitaldata to analog and then in turn to stream the analog data into anydesired 12-lead ECG device.

The following represents an optimized data format that may be used inconjunction with one possible embodiment of the present invention:

Header: Stored or other ECG data to be converted back to analog need nothave a header.

Sample value: Although this can theoretically vary without greatconsequence, the sample value in the utilized format is a 16-bit signedinteger, ranging from +2047 to −2048, in Intel byte order, meaning lowbyte first, or “little endian.”

Format: The DAC device presently assumes that the incoming digital datawill be in binary format, as one would obtain directly from amultiplexed ADC. In an embodiment, the specific format utilized ispresented on a sample-by-sample basis further below. However, in apreferred embodiment, the preferred digital format is one wherein allgiven precordial electrodes are referenced not to Wilson's CentralTerminal, but rather to a limb electrode, in an embodiment to the rightarm electrode, making preferred precordial channels the “CR” channelsrather than the “V” leads. Right arm electrode-referenced precordialchannels are ideal for a pre-DAC digital format because the repeat ADC,by any given 12-lead ECG device that follows DAC, may then naturallyconvert the so-formatted precordial channel data back to the Vprecordial lead format using whatever scheme the given ECG manufactureruses to accomplish that specific task for a traditional patient.

The Programming notes below provide further background information onhow a right arm electrode-referenced 12-lead ECG data format can beaccomplished in software and applied either to ADC, or, as in thisspecific embodiment, to an ideal digital format for pre-DAC.

Programming notes:

9+1 channels, e.g., from the electrodes on a patient:

EL, EF, ER, EC1, EC2, EC3, EC4, EC5, EC6 (+N)

EL: left arm

EF: left leg

ER: right arm

N: right leg (reference neutral)

ECi: chest (precordial) leads

Measured (raw data):

(8 channels): CL, CF, CR1, . . . , CR6

CL=EL−ER, CF=EF−ER, CR1=EC1−ER, . . . , CR6=EC6−ER

The combined data are mathematically represented as described hereinbelow:

I=EL−ER=CL

II=EF−ER=CF

III=EF−EL=(EF−ER)−(EL−ER)=CF−CL

aVR=ER−(EL+EF)/2=(2*ER−EL−EF)/2=−((EL−ER)+(EF−ER>>)/2=−(CL+CF)/2

aVL=EL−(EF+ER)/2=(2*EL−EF−ER)/2=(2*(EL−ER)−(EF−ER))/2=CL−CF/2

aVF=EF−(EL+ER)/2=(2*EF−EL−ER)/2=(2*(EF−ER)−(EL−ER))/2=CF−CL/2

Vi=ECi−(EL+EF+ER)/3=(3*ECi−EL−EF−ER)/3=(3*(ECi−ER)−(EL−ER)−(EF−ER))/3=(ECi−ER)−((EL−ER)+(EF−ER))/3=CRi−(CL+CF)/3

i=1 . . . 6

The binary data are thus constituted by I, II, and CR1-6 which representeight independent data channels. Furthermore, Vi=CRi−(I+II)/3 and thusCRi=Vi+(I+II)/3. In an embodiment, CRi=ECi if ER=0, which provides abasis for the optimized format used in the present system. Given theabove information and definitions, an optimized data format that isultimately input into the DAC device can be represented as follows:

Sample (0): Channel 1 = CL = Lead I (2-bytes) Bytes 0-1 Channel 2 = CF =Lead II (2-bytes) Bytes 2-3 Channel CR1 (2-bytes) Bytes 4-5 Channel CR2(2-bytes) Bytes 6-7 Channel CR3 (2-bytes) Bytes 8-9 Channel CR4(2-bytes) Bytes 10-11 Channel CR5 (2-bytes) Bytes 12-13 Channel CR6(2-bytes) Bytes 14-15 Sample (1): Channel 1 = CL = Lead I (2-bytes)Bytes 16-17 Channel 2 = CF = Lead II (2-bytes) Bytes 18-19 Channel CR1(2-bytes) Bytes 20-21 Channel CR2 (2-bytes) Bytes 22-23 Channel CR3(2-bytes) Bytes 24-25 Channel CR4 (2-bytes) Bytes 26-27 Channel CR5(2-bytes) Bytes 28-29 Channel CR6 (2-bytes) Bytes 30-31 Sample (n): Etc.

The configuration referenced above provides an optimized data formateasily transformed and reconfigured for use with a preferred embodimentof the present invention. Right arm electrode-referenced precordialchannels are ideal for a pre-DAC digital format because as describedherein, the repeat ADC by any given 12-lead ECG device that follows anappropriately configured DAC step may then naturally convert CRprecordial channel data back to the V precordial lead format by usingwhatever scheme the given ECG manufacturer uses to accomplish thatspecific task on the instrumented human. As a result, this format ispreferred for all ECG data before being sent to the receiving ECGmachine for transformation.

Turning back to FIG. 3, a schematic diagram of the data processingmethod associated with the ECG data in accord with a preferredembodiment of the present invention is depicted. Step 300 is comprisedof digital electrocardiogram data produced by an ECG machine, stored ona computer, and/or transmitted via a computer network. Step 310 convertsdigital data to optimally formatted digital data using software. In oneembodiment, software can be preprogrammed with relevant calibrationfactors to multiply with the digital data based on the generating ECGmachine utilized in producing the original digital data. Step 310 isresponsible for changing the reference electrode from WCT to a limbelectrode, as discussed in regards to FIG. 1 and FIG. 2.

At step 320, DAC hardware converts the optimally formatted digital data,which has since been calibrated at step 310, back to analog data. Next,in one possible embodiment, analog data may be passed to voltage dividerbox to adjust the voltages associated with the analog data such as by afactor of 1:1000. These steps allow the receiving ECG machine to readilydistinguish between the different electrode leads, given the voltagesproduced by the human body are small and nearly indiscernible withoutamplification. Finally at step 340, the receiving ECG machine reconvertsanalog data to digital data, after which the receiving ECG machine'sdiagnostic algorithm analyzes digital data for a clinical diagnosis.

FIG. 4 depicts one possible general hardware configuration for use inaccord with one possible embodiment of the present invention. Theinvention is not limited to this particular hardware and may comprisedifferent hardware configurations. In this configuration, Pandaboard ES404 is responsible for receiving the optimally formatted digitalinformation into the system, whether that is streaming data, storeddata, or data otherwise accessible by Pandaboard ES 404. Next, breakoutcircuits interface the voltage levels of the received digital data toproperly interface with digital to analog converter 402, because thevoltage levels should be amplified to return to analog signals. Rightarm electrode 406 is grounded to right leg electrode 408 at the voltagedivider box through resistor 412, which provides the receiving ECGmachine the substantially same impedance as would be provided forelectrodes positioned on the living creature.

While the present signals are not perfect reproductions, futuretechnological developments, both in DAC and in ADC technologies, willlikely further improve the quality of reproduction. In relation to this,it should be noted that the Cardiax and CorScience ADCs used in theaforementioned validation studies utilize, like the majority of othercommercially available ECG devices, known non-optimal methods of ADCsampling that implement “time interleaving”. Importantly, such methodsalone, whether they implement “round robin”, i.e. Cardiax, or“pseudo-simultaneous”, i.e. CorScience, sampling, these devices mayintroduce subtle distortions into any digitized data, and thennecessarily into the re-digitized data. Newer ECG devices introducedinto the market, including an upgraded Cardiax device, are able toemploy a more truly simultaneous method of ADC sampling by fasterprocessor chips; for instance, Texas Instruments' ADS 1298. Therefore,data collected on ECG devices employing such newer chips may, with evengreater fidelity, be re-convertible back to the original analog and thenagain to an even higher quality state of re-digitization when used inaccord with the present invention described herein.

As discussed hereinbefore, future systems like the one described hereinmay be able to more efficiently and accurately simulate the originalanalog data as technology improves, particularly when combined with ECGrecorders that employ true simultaneous sampling and higher samplingrates.

Detailed descriptions of the preferred embodiment are provided herein.It is to be understood, however, that the present invention may beembodied in various forms. Therefore, specific details disclosed hereinare not to be interpreted as limiting, but rather as a basis for theclaims and as a representative basis for teaching one skilled in the artto employ the present invention in virtually any appropriately detailedsystem, structure or manner.

The foregoing description of the preferred embodiments of the inventionhas been presented for purposes of illustration and description only. Itis not intended to be exhaustive, nor to limit the invention to theprecise form disclosed; and obviously many modifications and variationsare possible in light of the above teaching. Such modifications andvariations that may be apparent to a person skilled in the art areintended to be included within the scope of this invention as defined bythe accompanying claims.

What is claimed is:
 1. A method for digital to analog conversion,comprising the steps of: receiving digital information representative ofa plurality of independent signals, wherein each independent signalrepresents at least one of a plurality of electrical potentials (i)between at least one pair of a plurality of electrodes placed on aliving creature or (ii) between at least one individual electrode fromsaid plurality of electrodes placed on said living creature and apredetermined reference comprising an electrical resultant or potentialdifference between two or more predetermined electrodes from saidplurality of electrodes placed on said living creature; producing aplurality of analog outputs from said digital information wherein afirst analog output is designated as a common reference; and imposing apredetermined voltage on a second analog output with respect to saidcommon reference.
 2. The method of claim 1, further comprising the stepof: utilizing said plurality of analog outputs for substantiallyrecreating a plurality of original lead signals, wherein said pluralityof original lead signals was previously produced by placing saidplurality of electrodes on said living creature.
 3. The method of claim1, wherein said step of receiving digital information is comprised ofreceiving said digital information in a predetermined format.
 4. Themethod of claim 3, further comprising the step of: converting saiddigital information in said predetermined format to a predeterminedoptimal format.
 5. The method of claim 4, wherein said step ofconverting said digital information is comprised of assigning saidcommon reference to said first analog output and wherein said firstanalog output corresponds to a first electrode of said plurality ofelectrodes.
 6. The method of claim 5, wherein said first electrode ofsaid plurality of electrodes is an electrode placed on a limb of saidliving creature.
 7. The method of claim 6, wherein said plurality ofelectrodes comprises a plurality of limb electrodes and a plurality ofprecordial electrodes configured to produce an ECG recording.
 8. Themethod of claim 7, wherein said plurality of limb electrodes comprises aright arm electrode, a left arm electrode, a right leg electrode, and aleft leg electrode.
 9. The method of claim 6, wherein said second analogoutput corresponds to a second electrode of said plurality of electrodescomprising a second limb of said living creature.
 10. The method ofclaim 1, wherein said step of imposing a predetermined voltage comprisesimposing a zero voltage on said second analog output.
 11. The method ofclaim 10, wherein said imposing said zero voltage on said second analogoutput comprises electrically connecting said second analog output tosaid common reference.
 12. The method of claim 11, wherein saidelectrically connecting said second analog output to said commonreference comprises connecting said second analog output to said commonreference through a resistor utilized for impedance matching.
 13. Themethod of claim 1, wherein said plurality of electrodes comprises aplurality of limb electrodes and a plurality of precordial electrodesconfigured to produce an ECG recording, and wherein said plurality ofanalog outputs correspond to said plurality of limb electrodes and saidplurality of precordial electrodes.
 14. The method of claim 13, whereinsaid digital information is produced by a first ECG machine made by afirst manufacturer, and further comprising the step of: connecting saidplurality of analog outputs to a second ECG machine made by a secondmanufacturer.
 15. The method of claim 14, further comprising the stepof: comparing a first analysis of said ECG recording produced by saidfirst ECG machine to a second analysis of said ECG recording produced bysaid second ECG machine.
 16. The method of claim 13, further comprisingthe step of: utilizing said plurality of analog signals to produce aplurality of analyses from different ECG machine manufacturers.
 17. Anapparatus for recreating substantially original lead signals produced byplacing a plurality of electrodes on a living creature, comprising: aprocessor operable for receiving digital information representative of aplurality of independent lead signals, wherein each of said plurality ofindependent lead signal represents at least one of a plurality ofelectrical potentials (i) between at least one pair of said plurality ofelectrodes placed on said living creature or (ii) between at least oneindividual electrode from said plurality of electrodes placed on saidliving creature and a predetermined reference comprising an electricalresultant or potential difference between two or more predeterminedelectrodes from said plurality of electrodes placed on a livingcreature; and digital to analog circuitry comprised of a plurality ofanalog outputs wherein said plurality of analog outputs is comprised ofa first analog output designated as a common reference and a secondanalog output wherein a predetermined voltage is imposed on said secondanalog output, and wherein said substantially original lead signalscomprise electrical potentials between said plurality of analog outputs.18. The apparatus of claim 17, wherein said processor is operable forreceiving said digital information in a predetermined format.
 19. Theapparatus of claim 18, wherein said processor is operable for convertingsaid digital information in said predetermined format to a predeterminedoptimal format.
 20. The apparatus of claim 19, wherein said processor isoperable to control said digital to analog conversion circuitry suchthat said common reference is said first analog output, and wherein saidfirst analog output corresponds to a first electrode of said pluralityof electrodes.
 21. The apparatus of claim 19, wherein said digitalinformation is created based on utilizing two or more of said pluralityof independent lead signals as said predetermined reference and whereinsaid predetermined optimal format is based on said processor beingoperable for controlling said digital to analog conversion circuitry tochange said common reference to said first analog output.
 22. Theapparatus of claim 20, wherein said first electrode of said plurality ofelectrodes is placed on a first limb of said living creature.
 23. Theapparatus of claim 20, wherein said plurality of electrodes comprises aplurality of limb electrodes and a plurality of precordial electrodesconfigured to produce an ECG recording.
 24. The apparatus of claim 23,wherein said plurality of limb electrodes comprises a right armelectrode, a left arm electrode, a right leg electrode, and a left legelectrode.
 25. The apparatus of claim 23, wherein said second analogoutput corresponds to a second electrode of said plurality ofelectrodes, wherein said first electrode is placed on a first limb ofsaid living creature, and wherein said second electrode is placed on asecond limb of said living creature.
 26. The apparatus of claim 17,wherein said predetermined voltage on said second analog output withrespect to said common reference comprises a zero voltage.
 27. Theapparatus of claim 26, further comprising: a substantially directelectrical connection from said second analog output to said commonreference.
 28. The apparatus of claim 27, wherein said substantiallydirect electrical connection comprises a connection from said secondanalog output to said common reference and is made with a resistorutilized for impedance matching.
 29. The apparatus of claim 17, whereinsaid plurality of electrodes comprises a plurality of limb electrodesand a plurality of precordial electrodes configured to produce an ECGrecording, and said plurality of analog outputs correspond to saidplurality of limb electrodes and said plurality of precordialelectrodes.
 30. The apparatus of claim 29, wherein said digitalinformation is produced by a first ECG machine made by a firstmanufacturer, and further comprising: a plurality of connectionsconnecting said plurality of analog outputs to a second ECG machine madeby a second manufacturer.
 31. The apparatus of claim 30, wherein saidfirst ECG machine is operable for producing a first analysis of said ECGrecording, and said second ECG machine is operable for producing asecond analysis of said ECG recording.
 32. The apparatus of claim 29,wherein said plurality of analog outputs is connectable to produce aplurality of analyses from ECG machines of different manufacturers.